Chick limb mesoderm cells differentiate into one of three phenotypically distinct cell types; myoblast, chondroblast, and fibroblast. The goal of this research is to provide details of the controlling mechanisms governing this differentiation process. Our working hypothesis proposes that certain molecules common to all cell types can act to control phenotypic expression. This regulation can be accomplished by shifts in local concentrations of molecules essential for specific synthetic pathways. One such essential molecule, nicotinamide, has been shown to influence mesodermal cell expression. Its influence is being followed during several phases of mesoderm development in in vitro cell systems which closely mimic in vivo events. Teratogens antagonized by nicotinamide are being used in these in vitro systems since they affect either muscle or cartilage development in vivo. Both in vivo and in vitro effects of the teratogens are neutralized by co-treatment with nicotinamide. Several parameters are being monitored including analysis of pyridine nucleotides, selected metabolic pathways involving pyridine nucleotides, pyridine nucleotide related macromolecules associated with chromatin, and analyses of genetically altered phenotypic expression. Our experimentation emphasizes three areas: 1) The quantitation of pyridine nucleotides as a function of limb and limb cell development. 2) The quantitation of a unique macromolecule associated with chromatin and derived from the polymerization of NAD; this molecule, called polyADPR, may be a functional link between cytoplasmic NAD nuclear events; and 3) An analysis of chick limb mutant material which "overexpresses" limb properties. It is hoped that information acquired in these areas will lead to a better understanding of events controlling muscle and cartilage development in the limbs of developing chick embryos. The use of cell cultures and teratogens causing specific phenotypic lesions provides a unique approach for investigating the control of phenotypic expression.